This invention relates to an improved process for flocculating and thickening and/or dewatering aqueous pigment slurries containing 10% by weight or more of pigment, which provides improved dewatering and simultaneously provides improved redispersion of the pigment after filtering it. More particularly, this invention relates to the use of an anionic copolymer flocculant comprising at least about 1% 2-acrylamido-2-methyl propyl sulfonic acid and acrylamide for treating finely divided minerals such as kaolin clay, calcium carbonate, calcium sulfate, and the like, to purify, filter, dewater, redisperse, and prepare them for shipment.
Flocculants are reagents which are added to suspensions of solids to cause the solids to agglomerate (or floc) and settle or dewater more efficiently. A highly effective type of flocculant is the polymeric type which may be prepared in numerous variations. Polymeric water-soluble flocculants may be nonionic, anionic, cationic, or amphoteric. The particular type of flocculant to be used in a given application may depend upon the nature of the surface of the suspended solids and other factors such as pH.
Acrylamide monomer, a nonionic, is a basic building block for water soluble polymers because of its price and availability. It may be homopolymerized to obtain nonionic polymers. It is frequently copolymerized with one or more monomers containing primary, secondary, tertiary, or quaternary amine groups to obtain cationic flocculants. Similarly, it may be copolymerized with acrylic acid, 2-acrylamido-2-methyl propyl sulfonic acid and other monomers containing acid groups to obtain anionic flocculants.
Water soluble copolymers containing sulfonate groups are known in the art to be effective flocculants. U.S. Pat. No. 3,692,673 to Hoke teaches the use of polymers of 2-acrylamido-2-methyl propyl sulfonic acid and their salts as flocculants particularly for use in clarification of surface water and municipal sewage. While Hoke uses a low solids suspension of kaolin (200 ppm) to approximate surface water turbidity the present invention is different in that Hoke works with a 200 ppm suspension of kaolin whereas the present invention is concerned with 10% or more mineral slurries (100,000 ppm or more). Second, the objective of the work reported in Hoke is supernate clarity, i.e. improved solids removal and/or improved settling rate, whereas the present invention involves thickening and/or dewatering. Thickening or dewatering, i.e. the creation of a compact filter cake containing as little water as possible, places different and more stringent requirements on a flocculant than the separation of solids from water without regard to the condition of the filter cake. In U.S. Pat. No. 3,617,572 Monagle teaches the use of copolymers of acrylamide and an alkali metal salt of vinyl sulfonic acid as effective flocculants for improved settling of clays, including kaolin, which are present as impurities in concentrated salt solutions. Again, Monagle is concerned with improved solids capture, i.e. clarity or settling rate, which is distinct from the objective of the instant invention. Second, it is recognized in the art that vinyl sulfonate polymerizes very, very sluggishly compared to acrylamide. As a result, copolymers of vinyl sulfonic acid and acrylamide are very likely block-like. Conversely, 2-acrylamido-2-methyl propyl sulfonic acid (AMPS) and its salts polymerize at a rate only slightly less than that of acrylamide; acrylamide/AMPS copolymers are approximately random copolymers. Due to the significant difference in intra-molecular structure, any performance results of VSA/AM copolymers would not anticipate or predict the results to be obtained with AMPS/AM copolymers. Finally, in U.S. Pat. No. 4,342,653, Halverson teaches the use of copolymers of 99-65 mole percent acrylamide and 1-35 mole percent 2-acryl-amido-2-methyl propyl sulfonic acid and having Brookfield viscosities of at least 2.0 cps at 0.08% concentration in one normal NaCl as effective flocculants especially useful for treating phosphate slimes. Halverson also describes the use of AMPS/AM copolymers as flocculants for humate wastes, for processing streams encountered in the copper, uranium, potash and coal industries. Halverson describes the use of AMPS/AM polymers as coagulant aids, in conjunction with alum, for color removal. All the performance data reported by Halverson pertains to improved settling rate or reduction of the volume of the underflow.
The reader may also be interested in Engelhardt et al U.S. Pat. No. 4,357,245, which describes a drilling mud containing 2-acrylamido-2-methylpropane-3-sulfonic acid copolymerized with acrylamide, and the prior art reviewed therein. See also Engelhardt et al U.S. Pat. No. 4,309,523, also describing a drilling mud, and Lim et al U.S. Pat. Nos. 4,077,930 and 4,147,681 which describe self-inverting emulsions of similar polymers. Polymers similar to the ones I employ have also been used by Adams et al in U.S. Pat. No. Re. 29,595, Randin in U.S. Pat. No. 4,296,016, and Doggett et al U.S. Pat. No. 4,136,078 for various purposes.
Despite these and many other teachings, no one has yet developed a flocculant which will effectively flocculate aqueous pigment slurries such as Kaolin clays for dewatering or thickening and subsequently permit effective redispersion of the thickened or dewatered slurry to obtain a high solids, e.g. 60-70% or more solids, low viscosity product. This is not too surprising when one considers that flocculation and dispersion of solids in a slurry are totally opposite phenomena. A flocculant causes the particles to agglomerate into larger clumps which settle or dewater more rapidly. A dispersant functions by causing the individual particles to repel--the opposite mechanism.
Approximately 5 to 6 million tons of kaolin clay per year are mined, processed and shipped from Georgia alone. The clay is usually mined by the open pit method. It is then slurried in water to about 20-30% solids, dispersed with various combinations of inorganic and organic dispersants to maintain a low (almost water-like) viscosity, and processed. Processing includes removal of iron and other magnetic impurities, removal of sand and silt, bleaching for whiteness and brightness, and, finally, dewatering, usually in vacuum rotary filters, to obtain a filter cake which is 50-60% solids. Part of the filter cake is spray dried, packaged and sold as solid dry clay. A portion of the filter cake is spray dried and blended back with the remaining filter cake to obtain a 70% solids aqueous slurry. Additional dispersants may be added to the 70% slurry as needed to obtain a final Brookfield viscosity of less than 1000 cps (preferably less than 500 cps). This low viscosity 70% slurry is then shipped in bulk to the customer.
While many high molecular weight organic polymers--cationic, anionic and nonionic--have been shown to be capable of flocculating kaolin clay in a satisfactory manner, none has been able to do so without having deleterious effects in the blending back or redispersing of the 70% slurry. The presence of the flocculant in the slurry typically will work against the desired effect of the dispersant, typically a low molecular weight polyacrylic acid, to disperse the solids and lower the viscosity. The necessary low viscosity for the final product has not been achieved, and a solids content higher than 60% has not yet been attained, to my knowledge.